LDL receptor relative with 11 ligand-binding repeats (LR11) is a LDL receptor-analogous protein having a structure specific to LDL receptor family (Patent Document 1 and Non-Patent Document 1). It is known that, although hardly expressed in cells of normal blood vessel wall, the protein is recognized to be specifically expressed in thickened intimal smooth muscle cells (Non-Patent Document 2). It is also known that, according to the study using cultured smooth muscle cells, expression amount of LR11 increases in accordance with the proliferation of the smooth muscle cells, so that secretion of LR11 into a culture medium is recognized, and when LR11 gene is functionally impaired in a mouse model having defective cuff by using development engineering method, thickening of vascular intima caused by migration and proliferation of smooth muscle cells is inhibited (Non-Patent Document 3). Furthermore, it is recently found that, in addition to membrane-bound type LR11, a soluble LR11 which is fragmentized by a protease exists (Non-Patent Document 4). Still furthermore, it is reported that the soluble LR11 also exists in human blood (Non-Patent Document 5).
As a method of quantifying soluble LR11, a method in which soluble LR11 in a sample is separated by using an insoluble carrier to which a chaperone molecule RAP (receptor associated protein) having an affinity for LR11 is bonded and then the protein is subjected to SDS-PAGE and Western blot followed by detection based on immunostaining using an anti-LR11 antibody, etc. is known (Non-Patent Documents 5 and 6). However, the method requires a process of separating soluble LR11 from a sample, and therefore operation of the process is complicated.